1. Field of the Invention
The present invention relates to an automatic transmission for vehicles using a V-belt type continuously variable transmission, and more particularly to a hydraulic regulator for regulating the hydraulic pressure (line pressure) supplied to hydraulic servo system of the automatic transmission.
2. Description of the Prior Art
A V-belt type continuously variable transmission can be used in combination with a planetary gear set for changing forward and reverse drives as an automatic transmission for vehicles such as automobiles. The continuously variable transmission is controlled by an automatic control device comprising an electric control circuit receiving input signals of vehicle running conditions such as the throttle opening, the vehicle speed, the input pulley revolution speed or the shift lever setting position, and a hydraulic control circuit adjusting the reduction ratio between input and output shafts in the V-belt type continuously variable transmission and selecting the forward or reverse drive of the planetary gear set. The hydraulic control circuit is supplied with the line pressure produced by regulating with a hydraulic regulator the delivery pressure of an oil pump driven by an engine. The line pressure required by the hydraulic control circuit varies mainly depending on the reduction ratio between the input and output shafts of the V-belt type continuously variable transmission and the input torque from the engine. There is a minimum value of the line pressure needed for running without the belt slip-is prescribed.